The effects of endorphin and casomorphin on the behavior and function of steroid-producing glands were tested in 25 male baboons during free behavior and emotional stress caused by a single 2-hour immobilization. The stress-protective effect of peptide preparations was rated by the indices of the individual and social behavior of the monkeys, as well as by the level of corticosteroid and gonad systems. Exogenous opioids on the behavioral and neurohormonal manifestations of the stress-reaction in the baboons were found, which suggests that the use of regulatory peptides is promising as protective agents of emotional stress.